It is well-known in the art to incorporate a reed-relay on a hearing prosthesis circuit board and be operable to switch between a telecoil input signal and a microphone input signal depending upon a field strength value of an externally applied static magnetic field. The externally applied static magnetic field has in the prior art been generated by manual user intervention through use of a hand-held rod-shaped magnet actuator, or by a static magnetic field radiated by a loudspeaker magnet of a telephone handset when the latter approaches the hearing prosthesis.
U.S. Patent Application Publication No. 2003/0059073 discloses a hearing aid circuit that comprises a magnetically activated switch in the form of a reed-relay adapted to switch between a microphone and a telecoil input signal. The switching circuitry and the associated reed relay is a relatively complex construction utilizing a number of bipolar transistors and other circuitry and arranged on a circuit board of the hearing aid.
Several drawbacks are associated with these prior art techniques. The inclusion of, for example, a reed-relay and wiring associated therewith within the hearing prosthesis leads to a significant occupation of printed circuit board area or hybrid substrate area in the hearing aid. This is particularly troublesome for small hearing aids such as ITC and CIC-type of hearing aids which are adapted for positioning partly or entirely within the user's ear canal and which therefore must posses very small dimensions.
By routing the switchable input signals through the relay switch itself, it is impossible to dispense with or overrule the automatic switching between input signal sources when a static magnetic field of some predetermined threshold value is applied to the reed relay. The switching logic is integral to the magnetic sensing mechanism as such and thus unable to cooperate with, e.g., a hearing aid signal processor, such as a microprocessor or digital signal processor, to determine whether input signal source switching is needed.